Expandable thermoplastic microspheres comprising a thermoplastic polymer shell encapsulating a propellant are commercially available under the trademark EXPANCEL® and are used as a foaming agent in many different applications.
In such microspheres, the propellant is normally a liquid having a boiling temperature not higher than the softening temperature of the thermoplastic polymer shell. Upon heating, the propellant evaporates to increase the internal pressure at the same time as the shell softens, resulting in significant expansion of the microspheres. The temperature at which the expansion starts is called Tstart, while the temperature at which maximum expansion is reached is called Tmax. Expandable microspheres are marketed in various forms, e.g. as dry free flowing particles, as an aqueous slurry or as a partially dewatered wet-cake.
Expandable microspheres can be produced by polymerising ethylenically unsaturated monomers in the presence of a propellant. Usually the monomers mainly comprise monomers with one carbon-to-carbon double bond together with small amounts of cross-linking monomers having two or more carbon-to-carbon double bonds. Detailed descriptions of various expandable microspheres and their production can be found in, for example, U.S. Pat. Nos. 3,615,972, 3,945,956, 4,287,308, 5,536,756, 6,235,800, 6,235,394 and 6,509,384, 6,617,363 and 6,984,347, in US Patent Applications Publications US 2004/0176486 and 2005/0079352, in EP 486080, EP 1230975, EP 1288272, EP 1598405, EP 1811007 and EP 1964903, in WO 2002/096635, WO 2004/072160, WO 2007/091960, WO 2007/091961 and WO 2007/142593, and in JP Laid Open No. 1987-286534 and 2005-272633.
One important application for expandable microspheres is as a foaming agent for processing of polymeric materials, for example in injection moulding and extrusion moulding. In some cases it is desirable to have microspheres with a high expansion temperature in combination with high thermo stability after expansion, for example when microspheres are processed together with a polymer at high temperatures during a long time period. It is also desirable to have microspheres with a broad expansion window so that the microspheres can be used at different temperatures without having to reformulate the microsphere composition during processing. This would give the end user of the microspheres more freedom to design the process conditions when optimizing the foaming of a certain material.
EP 1508604, EP 1577359 and EP 1964903 disclose microspheres having high expansion temperature that are made by including significant amounts of methacrylic acid monomers in the polymer shell.
Even though microspheres with high expansion temperature can be obtained, methacrylic acid monomers render the production process complicated. For example, methacrylic acid is highly water soluble and the process conditions requires low pH, large amounts of salt in the water phase and water soluble inhibitors to prevent agglomerates and lumps in the polymerization slurry. Low pH and salt can cause problems with corrosion in the polymerization reactors. High amounts of salt also require a higher water phase/organic phase ratio in order to be able to dissolve the salt, lowering the amount of microspheres produced per batch.
It is an object of the invention to provide expandable microspheres with high expansion temperature.
It is another object of the invention to provide expandable microspheres with a broad expansion window.
It is yet another object of the invention to provide expanded microspheres having excellent volume retention at high temperature.